Keys and key signatures

A key is the note a piece treats as home, plus the scale built on it. A key signature is the shorthand at the start of every staff that tells you which notes are sharp or flat throughout.

Why the signature exists

The major scale pattern only lands on white keys from C. From G, the seventh note has to be F♯ or the pattern breaks. From D you need F♯ and C♯. Rather than mark those accidentals on every single note, notation states them once, at the left of the staff.

So a key signature is not decoration. It is a compression trick.

The order is fixed

Sharps always appear in the order F C G D A E B. Flats appear in exactly the reverse: B E A D G C F. Not convention for its own sake — it falls straight out of stacking fifths, which is what the circle of fifths draws.

Key signatures covers how to read one and how to name the key from it in a couple of seconds.

Relative and parallel keys

Every major key shares its signature with a minor key a minor third below. C major and A minor both have no sharps or flats. They use the same notes and sound completely different, because they treat different notes as home.

Parallel keys are a different relationship: same tonic, different signature. See relative and parallel keys.

Related

If the difference between a key and a scale is not obvious, start with key vs scale. To work out which key a piece is already in, see how to find the key of a song. To move a piece into a different key, see transposition. Every key signature is tabulated with its chords in the key reference.

If key signatures are the part that will not stick, how to memorize key signatures covers the mnemonics and which of them are worth the effort.